According to the report of Nippon Gakujutsu Shinkokai, 69th Material Processing Committee, First and Second section Joint Research Meeting (1990, August), generation amount of steel scrap as iron type scrap has increased with accumulation amount of steel materials at a rate of about 1,000,000 tons year by year and it is expected that the generation amount of market scrap (obsolete scrap) will be about 40,000,000 ton/year at the year of 2,000, which amounts to 45% for the production quantity of crude steel.
At present, most of market scrap is melted in electric furnaces. Particularly, in recent years, for making the varied kinds of products and for reducing the cost, a so-called mini-mill method has been adopted in which scrap or direct reduced iron for dilution of impurities are melted in electric furnaces, continuously casted and rolled.
On the other hand, in an integrated steel work, a technique of scrap melting without electric power for melting has been developed in order to cope with the increase of the scrap and to ensure iron sources corresponding to diversification of the production quantity of crude steel in the trend of concentration of blast furnaces.
In recent years, along with the trend of consumption intended for higher quality, market scrap of surface treated steel has increased more and one of them includes tinned iron scrap (referred to "can scrap" as market scrap).
Tin (hereinafter referred as Sn) is an element that can not be removed in the steel making step and if Sn remains by more than about 0.04% in the steel, the hot workability or toughness is reduced, so that several proposals have been made for Sn removing technique upon re-utilizing Sn plated steel sheet scrap as the iron source as described below.
Method A (refer to Japanese Patent Laid-Open Hei 4-198429 and Hei 4-198430)
For removing plated portions on the surface of steel sheets, Sn-plated steel sheet scrap is heated to 300-1200.degree. C. and treated in a sulfurizing atmosphere thereby transforming Sn into SnS and SnS is mechanically separated or evaporated. Then, using a furnace of a top and bottom blowing converter type, slag is formed on an iron bath such that the top blowing oxygen jet is not in direct contact with the iron bath and oxygen is top blown, in which carbon material containing an appropriate amount of S (sulfur) and the Sn-plated steel sheet scrap treated in the sulfurizing atmosphere are continuously charged and melted. Since the S potential at the oxygen blowing point is increased and SnS of a low boiling point (about 1230.degree. C.) is formed, evaporation of Sn preferentially to Fe (iron) is enabled to obtain a molten metal containing less than 0.05% by weight of the Sn content and, subsequently, low Sn molten steel is manufactured by the usual steel making processes.
Method B (refer to Japanese Patent Laid-Open Hei 5-9600)
Using a method of packing coke and scrap or scrap and iron ore in a layerous form into a vertical furnace, blowing a combustion sustaining gas from primary tuyeres and secondary tuyeres to each of packed layers thereby conducting melting and reduction (method disclosed in Japanese Patent Laid-Open Hei 1-290711), producing molten iron of low impurity content in a vertical furnace charged with scrap containing less impurities, producing molten iron of high impurity contents in another vertical furnace charged with scrap of high impurity contents such as can scrap (for example, 0.63 wt % Sn content). Then, molten iron manufactured by the two furnaces are mixed to obtain pig iron, for example, of 0.06 wt % Sn.
Method C (refer to Japanese Patent Laid-Open Hei 7-207313)
In this method, coke and scrap are packed in a layerous manner into a vertical furnace and a combustion sustaining gas is blown from primary tuyeres and secondary tuyeres to conduct melting. Tin-plated steel sheet scrap is used at least as a portion of the scrap, an average post combustion ratio is controlled to 50% or higher by adjusting the blowing amount of the combustion sustaining gas of the secondary tuyeres, thereby removing Sn in the form of SnO.sub.2 enriched dusts.
Japanese Patent Laid-Open Hei 5-9600, Hei 1-290711 and Hei 7-207313 are proposed by the present applicant or the present inventor.
By the way, each of the prior art described above has the following problems.
Method A:
It is necessary to provide a processing device in a sulfurizing atmosphere for removing Sn on the surface of scrap and the preliminary processing step is indispensable prior to the melting step. Furthermore, for suppressing evaporation of iron at the oxygen blowing point, slag has to be formed in a considerable amount in order that the top blowing oxygen jet is not in direct contact with the iron bath. In this case, since the absolute amount of FeO in the slag is increased, the iron yield is decreased and, in addition, a problem such as damage to the refractories may be increased. Further, since slag desulfurization is scarcely conducted but, on the contrary, since the molten iron is sulfurized, desulfurization processing is essential.
Method B:
This is a melting method using a vertical furnace having tuyeres, and this is not a regular Sn removing technique but, so to speak, a dilution method by mixing of two molten metals, so that it requires another furnace capable of simultaneous pig iron tapping. Accordingly, it is not efficient in term of installation cost and operation cost. Further, a post combustion ratio and a latent heat of the exhaust gas are not considered.
Method C:
This is a melting method using a vertical furnace having tuyeres and a regular Sn removing method. However, it is necessary to control the blowing amount of the combustion sustaining gas from the secondary tuyeres and control the average post combustion ratio to higher than 50%. Therefore, reducing of the heat generation amount of the exhaust gas is tending to lower the economical merit.